Blossom End Rot on Squash: What It Is and How to Stop It

There’s nothing more frustrating than seeing your beautiful squash almost ready to harvest… only to notice that ugly brown, leathery patch spreading across the bottom. If you’ve ever found your courgettes, pumpkins, or butternuts suddenly turning mushy at the blossom end, you’ve met the all-too-common garden problem known as blossom end rot.

It looks alarming, but don’t panic, it’s not a disease, it’s a sign your plant is stressed. The good news? Once you understand what causes blossom end rot, it’s easy to fix and even easier to prevent next season.

What Is Blossom End Rot?

Blossom end rot is a physiological disorder, not a fungus or pest, caused by a calcium uptake issue in the plant. It usually affects squash, courgettes, tomatoes, and peppers, and it shows up as brown, sunken, leathery patches at the blossom end of the fruit (the opposite end to the stem).

The plant isn’t short on calcium in the soil necessarily, it’s just unable to absorb it properly, often due to inconsistent watering or extreme heat. Think of it as the plant equivalent of dehydration leading to nutrient imbalance.

Symptoms and Key Identifiers

Spotting blossom end rot early can save a lot of disappointment later. Here’s what to look for:

• Brown, leathery patches forming at the blossom end of fruits
  
• The spot may start small and rapidly expand, turning black and sunken
  
• Fruit around the area may soften or collapse
  
• Plants often still look healthy and keep flowering and producing new fruits

Your key clue? That unmistakable brown patch at the fruit base, while the rest of the plant appears perfectly fine.

What Causes Blossom End Rot?

At its core, blossom end rot happens when calcium can’t reach developing fruits quickly enough. Calcium strengthens plant cell walls, so when it’s missing, the cells at the end of the fruit break down.

Common culprits include:

• Erratic watering – alternating between drought and soaking
  
• Hot weather or dry winds – which make water (and calcium) uptake harder
  
• Excess nitrogen fertiliser – which fuels leafy growth at the expense of fruit health
  
• Poor root development – from compacted or waterlogged soil

Even if your soil has plenty of calcium, if the roots can’t access it, blossom end rot will appear.

How to Fix and Prevent Blossom End Rot

Once a fruit shows symptoms, it won’t recover but you can stop new fruits from being affected by improving plant care.

Here’s how to get things back on track:

Keep Soil Evenly Moist

This is your number one defence. Squash are thirsty plants, and consistent moisture is key to preventing nutrient uptake problems.

Water deeply a few times a week rather than with little-and-often sprinkles, and mulch around the base with compost, straw, or leaf mould to lock in moisture. This keeps the roots cool and evenly hydrated, especially important during hot or windy spells.

If you’re growing in containers, check the soil daily. Pots dry out faster, making blossom end rot much more likely.

Avoid Excess Nitrogen

It’s tempting to give your plants a boost with fertiliser, but too much nitrogen encourages leafy growth at the expense of fruit health.Use a balanced or potassium-rich feed (like tomato fertiliser) instead of a high-nitrogen one. This supports fruit development and helps the plant maintain a healthy nutrient balance.

Add Calcium If Needed

If you suspect your soil is low in calcium, it’s worth testing and amending it. You can add calcium through garden lime or crushed eggshells, but only sparingly, too much can raise soil pH and make other nutrients less available.Alternatively, for a gentle boost, water plants occasionally with diluted milk or liquid seaweed feed, both of which help improve calcium uptake naturally.

Hand Pollinate Fruits

Sometimes, blossom end rot can persist even when conditions seem perfect. In that case, poor pollination may also play a part. Incomplete pollination can cause uneven fruit development, making the blossom end more vulnerable.

Use a soft paintbrush or your fingertip to transfer pollen from a male flower to a female one (the female has a tiny fruit behind the flower). Hand pollination can dramatically improve fruit quality and reduce deformities that encourage rot.

This is also a great time to link readers to your guide if you want to show them how to grow healthy squash right from seed.

Don’t Panic – Your Plants Will Recover

The first fruits are often the worst affected, especially during sudden heatwaves or early summer dry spells. Once watering stabilises and plants settle into rhythm, the problem usually disappears on its own.Just remove damaged fruits to help the plant focus energy on producing new, healthy ones. Within weeks, you’ll be back to harvesting perfect, glossy squash.

A Few Extra Tips to Prevent Blossom End Rot Next Season

• Start squash in rich, moisture-retentive soil with added organic matter
  
• Mulch early to prevent big moisture swings
  
• Water deeply during hot weather, especially when fruits are forming
  
• Avoid disturbing roots once plants are established, they hate being moved
  
• Keep an eye on pH, slightly acidic (6 – 6.5) soil is best for calcium absorption

By keeping your plants calm, cool, and evenly watered, you’ll save yourself a world of heartbreak and your squash will reward you handsomely.

Blossom end rot might look disastrous, but it’s really your plant’s way of asking for more balance. That brown, leathery patch at the bottom of your squash is simply a sign of stress not disease.

Keep your soil moist but not soggy, go easy on the nitrogen, and give your plants consistent care. Add a little calcium if needed and try hand pollinating to ensure top-quality fruits.

Before you know it, you’ll be harvesting perfect, unblemished squash again, proof that even the fussiest veg can bounce back with a little love and attention.